Many methods of medical imaging utilize high-energy electromagnetic radiations, such as X-rays, that can penetrate body tissues to create an image of internal structures, as well as implanted materials or tools that are inserted into the body. Common examples include X-ray imaging, fluoroscopy and computed axial tomography (a.k.a. CT or CAT scans). The ability to image structures and materials within the body depends in large part on the contrasting radiopacity of the different structures and materials. Radiopacity is the ability of a material to attenuate or block the passage of X-rays and other forms of electromagnetic radiation. Radiopacity of a material correlates closely with density and for a given material is proportional to the thickness. Often, a radiopaque dye, radiopaque filler material or radiopaque marker is used to enhance the radiopacity of structures or materials to make them more visible using X-ray imaging techniques. This is particularly important for imaging structures that are made of low density materials because they lack sufficient radiopacity to be visible by themselves using X-ray imaging techniques.
Polymer materials, such as those used in the making of polymeric vascular stents, generally have very low densities and therefore not enough radiopacity to be easily viewed inside the body using X-ray imaging techniques. The present invention concerns itself in particular with bioabsorbable or bioresorbable polymeric vascular stents. The terms bioabsorbable and bioresorbable are used interchangeably in the medical device industry to describe a material that, after implantation in the body, breaks down over time and is absorbed/resorbed by the surrounding tissues. Typical materials for bioabsorbable or bioresorbable stents include polylactic acid (PLA) and polyglycolic acid (PGA) polyglactin (PLAGA copolymer). Additional stent materials suitable for the present invention are described in U.S. Pat. No. 7,731,740. Where allowed, this and all patents and patent applications referred to herein are hereby incorporated by reference. In general, a polymer with a glass transition temperature (Tg) of at least 45° C. or greater is preferred.
Some of the previous approaches to adding radiopacity or radiopaque markers to vascular stents are described in the following patents and patent applications: U.S. Patent Application 2007/0156230 Dugan, U.S. Pat. No. 6,293,966 Frantzen; U.S. Pat. No. 6,245,103 Stinson; U.S. Pat. No. 7,914,571 Calisse; U.S. Patent Application 2009/0204203 Allen; U.S. Pat. No. 8,127,422 Wu; U.S. Patent Application 2008/0009939 Gueriguian; EP 0894481 Stinson; U.S. Pat. No. 7,473,417 Zeltinger; U.S. Pat. No. 6,652,579 Cox; U.S. Pat. No. 6,174,330 Stinson; U.S. Pat. No. 6,368,346 Jadhav; U.S. Pat. No. 7,553,325 Stinson; U.S. Pat. No. 6,991,647 Jadhav; U.S. Pat. No. 7,951,194 Gueriguian; U.S. Pat. No. 6,464,723 Callol; U.S. Pat. No. 6,635,082 Hossainy; and U.S. Pat. No. 5,725,572 Lam.
Generally, the methods described in these patent references are not suitable for application to bioabsorbable or bioresorbable polymeric vascular stents. Therefore, it would be highly desirable to provide a radiopaque marker and a method of applying the radiopaque marker to a bioabsorbable or bioresorbable polymeric vascular stent.